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Cummins 3.9L 4BT

A look at Cummins’ other “mighty” diesel engine.

Jun 3, 2020
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The Cummins 5.9L 6BT engine is legendary at this point. Hands down, no argument. As Michael Jordan is considered the NBA's G.O.A.T by hoop fans worldwide, the "12-valve Cummins" is held in equally high regard by diesel enthusiasts everywhere. However, as Cummins diesels go, the 3.9L 4BT is another engine in the family that is more of an unsung hero.
Based on the same inline-cylinder block logic as the 5.9L powerplant—with two fewer cylinders of course—the 4BT (which deciphers as 4-cylinder, B-Series, Turbocharged) debuted in 1983 with two-valves-per-cylinder and made 105hp and 265 lb-ft of torque.
Photo 2/9   |   010 4bt Cummins Engine Specs
The engine powered small equipment, buses, and the boxy step-type vans that are better known as "bread trucks," and in 1998, it received an update (to 16 valves) that jacked performance to a more robust 170 horses and 420 lb-ft of torque.
If performance is a concern (and with only 105 hp it should be), the pre-'98 engine is the one to use, as its fueling is anchored by the vaunted p7100 injection pump, which is easily modified to increase fuel volume and help a 4BT make more power. However, and sadly, both iterations of the I-4 turbodiesel are not intercooled, so a method for cooling the air charge, and addressing many other internal hard-part challenges have to be considered, if hopping up a 4BT is the plan. Bottom line? Making really big steam will be expensive.
Photo 3/9   |   004 4bt Cummins Engine Specs
Because of its very respectable torque output, the 3.9L Cummins 4BT engine really has made a mark in the off-road/rock-crawling scene and is a popular swap engine for Jeeps, Ford Broncos, Chevy Blazers, Toyota 4Runners, and other vehicles that get down off the pavement.
Our intent with this report is simply to give the small-but-mighty Cummins 4BT turbodiesel a little shine by providing a little history about the engine, as well as a breakdown of its hard-core stats (chart courtesy of cumminshub.com).